A fault current limiter (FCL) is an active or passive device used at high power levels for limiting excessive current in an electric circuit during a fault condition, such as shorting. The purpose of the FCL is to limit damage to electrical distribution equipment which would otherwise be damaged or destroyed by a sudden fault such as a lightning strike. An FCL operates by inserting a fixed, or predetermined, impedance into a circuit under fault conditions to limit the current.
One example of an FCL is a superconducting shielded core reaction (SSCR), which is a passive device consisting mainly of a closed iron core disposed within a superconducting tube around which is wound a copper coil. The copper coil is electrically connected to the circuit that is to be protected by the SSCR under fault conditions. The shielding capability of the superconducting tube keeps the inductance low under normal operating conditions. Under fault conditions, the large current in the copper coil exceeds the shielding capability of the superconducting tube resulting in a large, sudden increase in impedance because the iron core is no longer shielded from the copper coil by the superconducting tube. The electrical performance of SSCRs is well documented in the literature.
Because the FCL and SSCR are used primarily in electrical transmission and distribution systems, they are subject to alternating current (AC) losses. It is well known that a superconductor is not entirely lossless under AC conditions. These AC losses are due to a hysteristic characteristic of type II superconductors which is the type of conductor that is typically used in practical devices. This hysteristic characteristic is a result of flux pinning which causes the superconductor which has just experienced a cycle under AC conditions to not return to its original state, resulting in electromagnetic energy loss in its conversion to heat which is dissipated in the superconductor. This is the AC hysteresis loss. For practical application of high-Tc (critical temperature) superconductors, it is desirable to minimize these AC losses. There is also an economic incentive to operate any superconductor device as close as possible to its critical current so that the superconductor's volume can be reduced, which translates into a cost reduction for the device itself. Therefore, reducing AC losses is important in the operation of any superconducting device, where the device may be incorporated in various components such as transmission cables, motors, generators and FCLs.
The previously described SSCR device is not the only configuration that can be used in the aforementioned electrical components. For example, an SSCR wherein the copper coil is disposed inside, rather than outside, of the superconductor tube will operate equally as well as a FCL. In this case, the applied magnetic field generated by the current in the copper coil penetrates the superconductor tube from the inside radius towards the outside radius as the current is increased in the copper coil. The superconductor does not have to be a cylindrical tube, as it can take the form of plural superconductor rings stacked together with or without spacing between adjacent rings. One approach to the design of a SSCR is disclosed in U.S. Pat. No. 5,892,644 wherein first and second magnetically opposed, parallel connected copper coils are wound on a common ferromagnetic core. Another approach to a SSCR used in a fault detection and current control circuit is disclosed in U.S. patent application Ser. No. 10/015,373, wherein a SSCR in a secondary circuit is used to control current in a primary circuit such as in an electrical distribution system, where the SSCR has a variable impedance or is used in combination with a variable current source.
The present invention reduces AC losses in an SSCR used as a FCL by employing an innovative winding of the SSCR's copper coil. In the inventive SSCR, a copper coil is wound both inside and outside of the superconducting tube, or rings, with the outside and inside coils electrically connected in series and further connected to the circuit being protected from a fault condition. During normal operation, AC losses are smaller than either externally or internally wound coils having an equivalent number of turns. In a fault condition, the applied magnetic field penetrates the superconducting coil simultaneously both from inside and outside the superconducting tube, or rings, for substantially increasing the impedance in the circuit and limiting the fault current.